426 Foota. Strobell: A method of focussing in photouiicrography. XVIII, 4. 



Figure 2. The - 7 D. lens was replaced with — 8 D. Looking 

 through this lens, a slightly lower plane of the section is seen. The niicro- 

 meter screw was then turned up until the focus chosen for figure 1 was 

 brought into view. The focussing lens was removed and the photograph 

 taken. The - - 8 D. lens not being the right number for this magnifieation, 

 the negative did not show the focus selected, but a slightly higher plane. 

 (c. f. Nos. 1 and 2.) 



In figures 3, 4, 5 and 6 the sarne experiment was repeated. In each 

 photograph focussing with successively higher lenses (Nos. 9, 10, 11 and 

 12), raising the micrometer screw enough each tirae to secure the focus 

 shown in figure 1, which represents the focus selected in each case. 



In these six figures, the centrosome is seen increasing in distinctness 

 as the higher planes of the section are reached ; the chromosouies changing 

 their form with like mechanical precision. The photographs were taken 

 consecutively, without it being necessary to repeat any one of them, and 

 it seems to us they demonstrate the accuracy of this method of focussing. 



Figure 7. Same section as above, magnifieation 1000 diameters. Focus 

 was taken on the centrosome, using a — 5 D. lens, Zeiss 2 mm obj. pro- 

 jeetion ocular 4 — diaphraghm at — bellows draw 29 3 / 4 inches (75 - 5 cm). 



Figure 8. Magnifieation 450. Zeiss 4 mm obj. projeetion ocular 4, 

 diaphraghm at 0. 



Figure 9. Magnifieation 286. Same objeetive and ocular as for figure 8 

 with shorter draw of the bellows. These figures were take to show the 

 greater depth attained by the lower magnifieation. 



We ascertain the magnifieation by measuring the objeet with a Zeiss 

 micrometer eyepiece and 2 mm objeetive, then take the measurement of 

 the figure in microns and divide the latter by the former. On aecount of 

 the difYerence in magnifieation, dependent upon the adjustment of the pro- 

 jeetion ocular, whether the diaphraghm registers or 10, we find this the 

 only aecurate method. In marking the scale of magnifications on the 

 camera bars , a note is made of the adjustment of the projeetion ocular, 

 for no scale can be correct which ignores this factor. * 



The plates used were the Carbutt New Process , manufactured by 

 the Keystone Dry Plate Company, Philadephia. 



The Hopkins Company of New York reproduced the photographs by 

 the half tone process. If any of our readers wish to compare the repro- 

 duetions with the original Velox prints, the original plate may be obtained 

 on request. 



2 ) A comparison of measurements taken with the micrometer eyepiece, 

 showed the collar of the Zeiss 4 mm must be set at the highest point (23) 

 to make the measurements agree with those taken by the 8 mm and the 

 2 mm. 



[Eingegangen am 6. Januar 1902.] 



